An Observational Estimate of the Direct Response of the Cold-Season Atmospheric Circulation to the Arctic Sea Ice Loss

The direct response of the cold-season atmospheric circulation to the Arctic sea ice loss is estimated from observed sea ice concentration (SIC) and an atmospheric reanalysis, assuming that the atmospheric response to the long-term sea ice loss is the same as that to interannual pan-Arctic SIC fluctuations with identical spatial patterns. No large-scale relationship with previous interannual SIC fluctuations is found in October and November, but a negative North Atlantic Oscillation (NAO)/Arctic Oscillation follows the pan-Arctic SIC fluctuations from December to March. The signal is field significant in the stratosphere in December, and in the troposphere and tropopause thereafter. However, multiple regressions indicate that the stratospheric December signal is largely due to concomitant Siberian snow-cover anomalies. On the other hand, the tropospheric January-March NAO signals can be unambiguously attributed to SIC variability, with an Iceland high approaching 45m at 500 hPa, a 2 degrees C surface air warming in northeastern Canada, and a modulation of blocking activity in the North Atlantic sector. In March, a 1 degrees C northern Europe cooling is also attributed to SIC. An SIC impact on the warm Arctic-cold Eurasia pattern is only found in February in relation to January SIC. Extrapolating the most robust results suggests that, in the absence of other forcings, the SIC loss between 1979 and 2016 would have induced a 2 degrees-3 degrees C decade(-1) winter warming in northeastern North America and a 40-60 m decade(-1) increase in the height of the Iceland high, if linearity and perpetual winter conditions could be assumed.